1. Field of the Disclosure
This disclosure relates generally to non-volatile memories (NVMs), and more specifically, to NVMs that perform a variable number of verify operations.
2. Description of the Related Art
Typical non-volatile memories (NVMs) use charge pumps for generating erase voltages to apply to the memory cells during program and erase operations. The voltages selected for program and erase operations are based on desirable characteristics for the threshold voltage distribution of the memory cells and the program and erase performance of the cells that are programmed or erased. For a given NVM, there is a determined charge pump voltage for program and erase for each relevant node. For erase this is typically the gate voltage and the well voltage on the memory cells. For programming this is typically the gate voltage and the drain voltage on the memory cells.
The operational cycles can include modes such as pre-program, erase, compaction and soft program during which a series of voltage pulses are applied to the memory cells. Each of the operational cycles can include a verify operation to ensure that a specified number of the memory cells are in the intended state. The verify operations are performed after each series of voltage pulses are applied. If the cells do not pass verify, then another series of voltage pulses which could be at a higher voltage level, depending on the specific operation being performed, is applied until the specified number of cells are in the intended state. The number of voltage pulse series that are applied typically increases with the lifecycle age of the memory array. The increase in the number of pulse series affects the amount of time required to complete an operation. In addition, temperature will also impact the number of pulse series, with the number of pulse series increasing proportionally with age. The number of erase pulse series decreases with temperature. The number of program pulse series increases with temperature.
Thus there is a need for erase operations that improve erase performance and reliability over the life of the memory and erase time.